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%I #14 Feb 11 2024 12:49:05
%S 3,7,11,3,19,23,3,31,5,3,43,47,3,5,59,3,67,71,3,79,83,3,7,5,3,103,107,
%T 3,5,7,3,127,131,3,139,11,3,151,5,3,163,167,3,5,179,3,11,191,3,199,7,
%U 3,211,5,3,223,227,3,5,239,3,13,251,3,7,263,3,271,5,3,283,7,3,5,13,3,307
%N Smallest prime divisor of 4n-1.
%C All divisors of 4n-1 are odd.
%H Nick Hobson, <a href="/A125255/b125255.txt">Table of n, a(n) for n = 1..1000</a>
%F a(3k+1) = 3, k = 0,1,2,... a(5k+4) = 5 if k is not a multiple of 3; k = 1,2,4,5,7,8,... - _Alexander Adamchuk_, Nov 28 2006
%F a(n) = A020639(4n-1). - _R. J. Mathar_, Jan 23 2007
%e The prime divisors of 4*9 - 1 = 35 are 5 and 7, so a(9) = 5.
%o (PARI) vector(77, n, factor(4*n-1)[1,1])
%Y Cf. A004767, A107745, A125254.
%K easy,nonn
%O 1,1
%A _Nick Hobson_, Nov 26 2006
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